1. Field of the Invention
The invention relates to adjustable telescopic devices of the kind designed for substantially vertical mounting and having an inner tube which is slidably inserted in an outer tube and projects upwards from the same and which is adapted to carry a load and near its lower end can be locked to the outer tube. The invention has been developed with a view to being used in spindles for adjustable working chairs, that is where the load is constituted by the weight of the user with the addition of that of the seat and possible the back of the chair, but may also be contemplated to be used in several other situations such as for adjustable table legs, scaffold legs, stays for agricultural implements etc. where a height adjustment is required.
The possibility of adjusting the height of working chairs is very import in many situations, especially where the same working place is used by different persons at different times, since a correct sitting level is important for the physiology and comfort of the user. But in order that this possibility is really constantly utilized when the demand arises, it is necessary that the adjusting system can be operated quickly and from seated position even by persons without special technical insight.
2. Description of the Prior Art
For this purpose telescopic devices of the kind referred to are frequently used, and a problem in that connection is to combine the possibility of easy operation with safe locking in adjusted positions. In many telescopic devices the locking takes place by some form by wedging action by using axially movable spring loaded wedges which either press tongues of the inner tube outwards towards the outer tube or are wedged between inner and outer tube. However, here a difficulty resides in that, although a small wedge angle ensures safe locking, it requires a great force for the release, whereas with greater wedging angles the locking becomes less safe. The use of wedge pieces between the outer tube and downwardly tapering faces on the inner tube has been mentioned already in the German Patent Specification No. 130 366 from 1901, but that specification also describes and shows a different possibility, i.e. to replace the wedge pieces by balls actuated from below by a spring loaded disc. With this device locking will take place by wedging of the balls under the spring load and be the more firm the greater the weight resting on the seat, whereas for adjusting movements in retracted position of the disc, only rolling friction has to be overcome.
A drawback in that connection is, however, that under heavy load from the weight of the user the balls will exert a heavy punctual strain on the outer tube by outward forces with the result that the latter can be deformed and gradually become permanently damaged, so that the function becomes unsatisfactory and operation from seated position will no more be possible.